A story / question relayed to me by my sister in law...
One of her friends lived in an area that had no eruv. She had heard that men were allowed to wear key tie tacks on Shabbat as a way to "carry" the house key as the key tie tack was now considered a piece of jewelry and one is allowed to wear jewelry on Shabbat.
So, she made a pin on the back of her key, and wore the key pin. Her rav said, no, this doesn't work for a woman, since a key pin is not considered a piece of jewelry. So, she placed a diamond on the pin above the key and now wore the pin, assuming that the diamond, being a jewel, now turned this into an actual pin and now made it the status of "jewelry". (My sister in law didn't relate the end of the story as to whether the rav had approved of this idea, or not.)
It's a standard (gold / silver) house lock key. A diamond fills the place where the hole on the key would be (i.e. the hole where a key chain or ring would usually be slipped.) The back side of the key has a pin with a clasp, similar to a large safety pin.
Is adding a gem stone to the key pin valid enough to make this jewelry and allow a woman to wear a house key on Shabbat?